Mechanical actuators with pistons are widely used in industry for moving parts or components of machinery to carry out various functions. Actuators are used in assembly lines or industrial processes to control valves, or to operate equipment. Actuators usually operate using pneumatic or low pressure hydraulic fluid to create a force, linear or rotary, to move a component or piece of machinery.
Pneumatic pistons or actuators are of two basic types:
A. Bellows or Diaphragm. These typically are hollow and consist of preformed rubber which extends and contracts in a linear manner by an “accordion” mechanism extending or collapsing the elastomer. To avoid radial bulging, the rubber must be very heavy, horizontal movement must be very short in relation to the radial dimension of the accordion shape, and pneumatic pressure must be sufficiently low so as not to rupture the rubber. Bellows type pistons are useful primarily for short thrust, low pressure movements such as switch or brake activation. Typical maximum working pressures of bellows type pistons are limited to about 20 psig.
B. Solid tube pistons. These actuators typically comprise a solid piston sliding within a hollow solid (usually metal) tube. Solid tube pistons typically operate at working pressures in the range of about 80 psig. To contain the required pneumatic force on the piston, one or more rubber air seals enclose the circumference of the piston and thereby contain the air. The air seals are similar to piston rings in an internal combustion engine. Typically, since the piston moves along the axis of the interior of the tubular cylinder, a linear force is generated. The term “actuator” is often applied in situations where a rotary (torque) force is to be generated. In the case of mechanical actuators, the rotational force is usually obtained by utilizing a rack and pinion arrangement within the cylinder. The rack is attached to the piston and the pinion exits the cylinder radially. This requires a seal (an O-ring, for example) to contain the air pressure. Various types of actuators are available, for example, double action and spring return.
The sliding piston in a fixed cylinder actuator is commonly used for applications such as valve stem rotation. The inherent problem with this type is that they are expensive to manufacture and have wear and friction problems associated with the necessity for sliding seals on the pistons. Contaminated air can significantly shorten the life of the seals, and the design of such actuators does not permit economical serviceability. Some applications therefore require the air to be filtered or otherwise treated to prolong actuator service life.
Other linear movement mechanisms exist which comprise a tube that stretches in a linear manner, such as for air ducting used in ventilation systems. These stretchable tubular mechanisms include plastic tubing with embedded coiled wire which allows horizontal stretch of the tubing. The coiled wire provides radial strength. There is an inherent problem with such tubes. When a high pneumatic pressure is applied to the tube, it tends to turn and cause localized bulging. Such tubes with internal or embedded coils are thus suitable only for very low pressure applications.
Various inventors have attempted to solve the problems inherent in the designs of these two types of actuators by using a sealed rubber tube (air bag) and restraining its radial expansion by various means other than a bellows. These systems generally involve surrounding the rubber tube with an outer tube having helical wires. This allows the outside tube to stretch without bulging. Another method utilizes a second outside tube with compensating pneumatic pressure. These systems generally shorten the available stroke of the actuator relative to its length and also set up counteracting forces which significantly decrease the mechanical efficiency of the expanding inner tube.
A number of patents have issued or been applied to increase the pressure holding capabilities of bellows type pistons for use in actuation:
Beullens—U.S. Pat. No. 4,841,845
Negishi—U.S. Pat. No. 5,201,262
Negishi—U.S. Pat. No. 5,158,005
Negishi—U.S. Pat. No. 5,067,390
Sakaguchi—U.S. Pat. No. 4,860,639
Takagi—U.S. Pat. No. 4,615,260
Wang—U.S. Pat. No. 4,833,973
Paynter—U.S. Pat. No. 4,108,050
Vergenet—U.S. Pat. No. 4,008,008
Larsson—U.S. Pat. No. 4,777,868
Price—U.S. Pat. No. 4,006,669
Perez—WO 00/03144
Perez, PCT application No. PCT/CA99/00616, filed 7 Jul. 1999 and published under WO 00/03144, 20 Jan. 2000, uses a tube made of a strong collapsible tubular fabric or similar material, in combination with an internal sealing device, such as an inner tube. The combination radially contains the pressure exerted by a fluid pumped into the interior of the combination, with the result that the fluid pressure is translated to a linear force exerted along the axis of the tube.
Eggleston—U.S. Pat. No. 6,000,675
Eggleston discloses a rotary valve actuator with movable actuator linkage maintained in a constant “pull-pull” tension. It includes a sliding canister, rotatable lever, a rolling diaphragm bladder and a return spring substantially aligned with each other. The linear motion of the canister in response to an expanding and contracting pressurized bladder is converted through chain linkage into rotary lever motion. The return spring is connected through respective chain linkage to the rotatable lever so the pulling tension of the spring/lever linkage rotates the rotatable lever and maintains tension on both chain linkages. Adjustable travel stops are provided. In an alternate embodiment, the return spring chain linkage is configured at right angles to the linear movement of the canister.